Technological features of metal welding depending on the material
Welding of metals was invented by people a little later than metalworking itself. Already the ancient blacksmiths knew how to create a single whole from individual iron products. Today welding is an integral part of modern civilization.
It is used in production, in everyday life, in the manufacture of building structures and cars, mechanical parts, and jewelry. Performed underwater and in outer space.
Modern technologies make it possible to achieve such strength in the resulting product as if it was originally solid.
The weldability of metals is studied by specialized institutes around the world, and welding is one of the most in-demand working professions today.
Basic Process Concepts
The task of welding is to create a permanent connection of parts . To obtain it, it is necessary to move the atoms that form the metal so close that interatomic cohesive forces begin to act. Why this happens, what physical processes occur in this case, has been studied rather poorly, but the property itself has been used for a long time.
Connections are achieved using three methods:
- Thermal , melting the metal of the parts themselves, or an additional additive. An example is the widespread arc and gas, laser, plasma jet. This includes soldering.
- Thermomechanical , in which products are heated and then compressed or forged. This is the most ancient method, which was used by blacksmiths already in the fifth millennium BC. In addition to forging, these are diffusion welding, contact welding, and induction-press welding.
- Using the mechanical method, products are welded, subjected to various influences without special heating. Example - ultrasonic techniques, connections by explosion, friction, pressure.
The first type is most common at construction sites and in everyday life. It is used to connect steel, cast iron, and to weld non-ferrous metals and alloys.
The second and third methods are used on industrial lines, for serial production, and also for the manufacture of unique products.
Regardless of the technique used, there are only two types of process:
- Seamless , when the metal of the parts themselves is connected directly.
- With the formation of a seam . In this case, auxiliary additives are used. They can either exactly match the base metal or differ. Example - joining steel with a copper electrode, soldering with tin, all types of welding using a fusible electrode.
Story
Until the second half of the 19th century, the only method of welded joints was forging, as well as soldering . The blacksmiths heated and placed the workpieces on top of each other, after which they beat them with hammers. Of course, there was no need to talk about reliability and accuracy.
The real revolution in welding processes began with the discovery of the electric arc process. Russian engineers were able to use it to connect parts. The method of arc welding with carbon electrodes was invented by N.N. Benardos, and literally a few months later, N. G. Slavyanov managed to perform the first connection of metals with a consumable electrode.
The 20th century was marked by a real welding boom. French engineers invented gas welding; in 1956, turner A.I. Chudikov managed to connect parts rotating on a lathe by friction. By the late 60s, people learned how to laser weld metal. Around the same time, industrial plasma spraying devices, induction and diffusion welding, etc. appeared.
Basic methods
Today there are more than 150 different welding methods. Although they differ in detail, they all fall into several main categories, which we will briefly consider.
Electric arc
The most common technology today . It is divided into two main types: non-consumable and fusible electrode.
Both of them are based on the temperature effect of a voltaic arc. In the first case, the problem of how to heat metal by electric welding is solved using a refractory tungsten electrode. It itself does not melt, but only forms an arc, forming a weld pool from the material of the parts themselves or with the addition of an additive.
The latter is introduced by hand, like a welding rod, or by automatic feeding, like a thin wire.
In the second option, an arc is formed between the metal being welded and the consumable electrode. The latter simultaneously serves to form a seam, which is formed by jet transfer of its metal.
To concentrate the arc, as well as to form a protective layer, the metal rod is coated with a layer of special coating. A number of components are also added to the coating, the transfer of which improves the quality of the seam.
To protect the melt from oxidation, a gas flow is supplied to the welding zone through the torch. Depending on its properties, they are distinguished:
- Welding in carbon dioxide . As a rule, they are driven by semi-automatic inverters. The speed of semi-automatic welding, depending on the thickness of the metal, dictates the required volume of carbon dioxide supply. Thin sheet metal is welded in this way, say, when repairing a car body.
- A compound in an environment of inert gases argon or helium . For example, pipelines made of stainless steel, aluminum, and special alloys are welded this way.
- Active gas welding.
The equipment used to carry out the work differs in design, but as a rule these are various types of transformers and generators.
The arc welding technique is universal; it allows both welding thin metal semi-automatically and connecting thick-walled steel pipes manually.
Automatic submerged
Welding flux is a powder made from various substances . Its task is twofold:
- Protect the molten metal from oxidation.
- Add substances to the joint to enhance the strength of the joint.
The process goes as follows.
Pre-prepared parts are fixed on a special stand. After this, a layer of flux is applied to the joint using a special tube, forming a long mound. Behind it comes the welding head, which forms an arc. This may be followed by another tube that sucks in unused flux powder.
The work is carried out with a non-consumable or consumable electrode. The flux partially melts. How to distinguish metal from slag during welding is not a task: after completing the work, it is enough to tap the seam so that the slag crust formed by the flux melt bounces off.
Electroslag
While the submerged arc technology is similar, the process is fundamentally different. The temperature here is created by a melt of a special composition, which, in addition to metal, includes conductive slag .
The melt is maintained by an electric current passing through it. In this case, heating is carried out immediately along the entire length of the connection.
The metal forming the connecting seam is also a fusible electrode that supplies current to the melt. This can be a wire, a plate, or a tubular mouthpiece.
Gradual heating and the same smooth cooling eliminates overheating of the product and does not create additional stress.
The method requires specific equipment and is performed in a factory environment . But he can weld thick metal for the entire array of parts at once.
Electron beam and plasma, diffusion and contact
While the methods and principles of connecting products differ, all these types are united by the absence of a voltaic arc (or a special type of it, as in plasma welding). Moreover, with contact technology, an arc is undesirable.
Electron beam joining of parts was first achieved back in the 50s. The process is similar to laser joining, but instead of a beam of photons, there is a directed beam of electrons. In this way, it is possible to connect tiny parts, the dimensions of which are measured in fractions of a millimeter, or to weld sheet metal measuring tens of square meters.
Plasma welding is suitable for working with refractory metals. Unlike an electric arc, whose temperature is 6-7 thousand degrees Celsius, in a plasma flow the heat reaches 30,000ºC. In addition to joining, the process allows for sputtering of dissimilar metals.
Diffuse welding is based on the interpenetration of metal atoms of tightly compressed, preheated parts. This process, called diffusion, gives the method its name. The method is effective in difficult cases, for example, it is used to weld non-ferrous metals.
Contact welding also refers to pressure joining. When parts are compressed, an electrical impulse passes through the contact point. current that provides communication.
Cold welding of parts and induction method
The basis of cold welding: deformation of the parts being joined . Otherwise - cold forging. Despite the attractiveness of the method (no electric current), it can be used to connect only non-oxidizing parts.
The induction method is the exact opposite of the cold method . The parts are heated, not with a flame or arc, but with a high-frequency electromagnetic field. In this case, welded metal products are heated to a significant depth.
The method is technologically advanced, so it is used, for example, on automated ones. pipe rolling lines.
How are different metals and alloys welded?
A special quality of metals is the concept of weldability, that is, the ability to be joined by welding using one or another method . It can depend on many reasons: chemical composition, specific properties of the metal or alloy, methods for preparing parts for welding, and much more.
GOST divides this property into the following weldable groups:
- Fine;
- satisfactorily;
- limited;
- Badly.
Cast iron, aluminum, and high-alloy steels have poor weldability. There are two main obstacles:
- oxide film preventing contact;
- the presence of alloying additives, in particular carbon, making the seam brittle and weak.
Weldability is improved by preliminary specific preparation of the metal for welding.
Non-ferrous metals are especially difficult to weld, each with its own specific characteristics..
Procedure
Although the technology for creating welded joints is different, in all cases the process begins with preparatory operations. For arc methods, this is preparing edges for welding, or, as they say, cutting them. For cold forging - thorough cleaning (even polishing) of parts to be joined, etc. Preparation may include cleaning from dirt and degreasing.
Another negative phenomenon that unites all types of welding contacts is temperature deformation and stress.
They deal with this in different ways. In some cases, products are fixed in advance with clamps. Another common method is heat treatment of the finished product: heating followed by slow cooling (tempering).
Finally
We have not considered all types and methods of welding. In addition, existing technologies are constantly being developed and new ones are being invented.
At the same time, technological solutions are dictated by tasks. In one case, the question is how to properly weld a metal bypass for an apartment heating system using the simplest manual method, in another - to create an ultra-precise product for the aircraft industry. Accordingly, a different approach is required.
The existing ones will inevitably be replaced by new methods, faster and more reliable.
Source: https://svarka.guru/tehnika/opredelennih-metallov/osobennosti.html
How to distinguish slag from metal when welding
» Articles » How to distinguish slag from metal when welding
New welders face many problems and questions that arise during the process. So, at the first stages, many find it difficult to distinguish slag from metal, and how to expel this slag during welding. In this article we will talk about how to do this and what is needed to ensure a high-quality weld.
The main difference between melting metal and slag during welding is its fluidity. The metal is more liquid and mobile, and during the welding process itself you can clearly see how it boils in the weld pool. In turn, slag is more viscous and has a darker shade.
Situations often arise when slag floats onto the weld pool. In such a situation, the welder can hardly see anything, and to avoid this, he has to tilt the electrode in such a way that the gas stream (which evaporates from the coating) blows the slag back onto the weld. After this, the weld pool opens again and the welding site becomes clearly visible.
To draw an analogy, we can give the example of a basin of soapy water. To see the water through the foam, you need to blow it off and only then will you have a view of the water and all the contents in it.
Thus, a novice welder always needs to keep one rule in mind - under no circumstances should slag run into the weld pool, and then the quality of welds will be much better.
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Welding slag as a prerequisite for the occurrence of slag inclusions
The main side effect of arc welding is welding slag. It is a by-product of non-metallic origin, which consists of either the molten electronic coating of the consumable electrode (in manual arc welding, including inverter welding) or molten welding flux (automatic submerged arc welding).
This material is a by-product because it must be removed after direct connection.
In the case when, for some reason, it begins to come into contact with the hardening part and becomes part of the resulting seam, this is a serious defect. This result is called slag inclusions.
They are visually visible on the surface of the product. You can remove slag inclusions when they are already completely in the hardened part only by drilling, and then weld it again.
The reason for the occurrence of slag inclusions is a situation when a small volume of metal hardens too quickly, as a result of which all the slag does not have time to “exit” beyond the weld pool area. If this defect is present, the connection will not be able to be used for its intended purpose.
Slag inclusions can be macroscopic or microscopic. The first ones appear in case of poor edge cleaning or its absence at all. They are a spherical material with elongated tails. Their occurrence can be avoided by thoroughly cleaning the edges of the parts being connected. Microscopic inclusions can arise during chemical reactions during the welding process, when the metal crystallizes.
Complete elimination of such manufacturing defects is impossible; with minor slag inclusions, the connection can be considered high quality.
There are certain conditions and signs under which the acceptable value of the presence of this type of defect on a product is established.
Such tolerances are established depending on the number, location and size of the defective seam; from the percentage ratio of the area of the entire defect to the area of deposited metal on the product; on the specific gravity of the deposited metal.
Reasons why a defect must be removed after work
- When removing slag, the product visually looks better.
- When many layers are produced.
- Often the product needs to be coated on top, for example with paint.
- To check how well the seam is made.
The main reasons for the appearance of slag inclusions
- Rapid solidification of small volumes of metal, as a result of which the slag does not have time to go beyond the boundaries of the weld pool.
- The use of electrodes and flux of high specific gravity and/or from refractory materials.
- Low rates of metal deoxidation. Deoxidation involves the process of removing oxygen molecules from an already soft metal.
Oxygen is a harmful component for it, which deteriorates the quality.
- High surface tension force of slag. At the same time, it does not float to the surface.
- The edges of the parts or seam beads are poorly cleaned.
- Poor quality of the electrode, or rather its coating, which melts unevenly; its particles end up in the weld pool.
- Failure to comply with the rules and techniques and modes of connecting parts (choosing the wrong speed, angle of inclination of the electrode), changing the arc length for no reason.
- Unprofessional welder. If you need a quality product, it is better to involve a specialist in this field in this difficult task.
If you want to do everything yourself, then before you take on important, complex work, you need to gain practice with simple connections.
This is what welding slag looks like after being removed from a seam.
Professional welders are able to immediately distinguish slag from metal during welding and “drive” it out during the work process, however, everyone has their own advice on how to do this.
Some argue that it is better to use new electrodes in which the coating is darker and the metal is red (this does not apply to rutile electrodes), others say that the metal is more liquid and the slag is viscous. Its viscosity is affected by temperature.
To prevent the slag from covering the weld pool, it is necessary to adjust the position of the electrode. The position must be such that the direction of the gas from the evaporation of the electrode coating blows this defect onto the surface of the seam. Under no circumstances should the welder leave it in the weld pool. Welding slag should quickly succumb to the crystallization process and be removed without much effort.
It is inevitable that slag gets into the finished joint; such a defect, if not impossible to eliminate, is often very difficult. That is why there are acceptable standards for the presence of “extra” inclusions in an already welded product.
For example, in the interstate standard regarding building steel structures (put into effect in 2001). The annex to this document provides requirements for the quality of welded joints and permissible defects.
The requirements for slag inclusions are shown in the table below.
| Long defects | Not allowed | ||
| Short defects: | |||
| butt weld | h ≤ 0.2 S | h ≤ 0.25 S | h ≤ 0.3 S |
| fillet weld | h ≤ 0.2 K | h ≤ 0.25 K | h ≤ 0.3 K |
| Maximum inclusion size | 2 mm | 3 mm | 4 mm |
Thus, to ensure that slag appears correctly on the surface of the weld, you need to know how to distinguish it from the metal. If it is noticeable that the slag remains in the weld pool and does not come up, you need to change the angle of the electrode.
Before the joining process, it is necessary to take care of the proper condition of the edges, as well as the correct choice of modes and parameters. Electrodes must be chosen of high quality, then the weld will be of high quality. If all conditions are met, then the slag will be non-viscous, of low specific gravity, with low surface tension.
Only in this case, slag compounds will interact with the part, increasing its deoxidation rates, removing oxygen. And only then will the welding slag easily come out to the surface of the seam. Here you cannot do without the professionalism of the welder performing the work.
He must be able to distinguish slag from metal during welding and know why it appears in the first place. Only an experienced specialist will be able to make a high-quality and durable connection.
Source: https://samsvar.ru/stati/kak-otlichit-shlak-ot-metalla-pri-svarke.html
How to distinguish slag from metal when welding and how to expel it
How to distinguish slag from metal when welding and how to expel it
When welding, beginners are faced with a lot of questions, one of which is how to distinguish slag from metal. Slag differs from metal not only in color and density, but also in fluidity. However, when you learn to weld, it is very difficult for an inexperienced welder to fully control the welding process, monitor the arc and the movement of the electrode, and even pay attention to the slag.
However, having learned to ignite an arc and guide the electrode, you will still have to deal with this issue, because in the welding process you need to not only distinguish where the metal is and where the slag is, but also be able to drive it out of the weld pool. This article from the website about welding mmasvarka.ru will talk about how to distinguish slag from metal and expel it during welding.
What causes slag to appear on metal?
Surely you have noticed more than once that right above the weld seam and along its edges, immediately after welding, a dark, highly porous crust forms, which easily flies off the metal when hit with a hammer. This is the very notorious slag - nothing more than a product of burnt electrode coating and oxidized metal.
Under no circumstances should you think that slag is evil. Of course not, because its main task is to block the access of oxygen to the weld pool, and thereby protect the metal from oxidation. However, if the slag gets directly into the weld itself, then there will be trouble. When slag gets into the weld pool, it seriously weakens the strength of the welded joint and can cause its destruction in the future.
How to distinguish slag from metal when welding
To distinguish slag from hot metal during the welding process, just remember the following:
- The slag is darker in color than the heated metal;
- The fluidity of the metal is much higher than that of slag;
- Slag and metal have different densities;
- Slag cools much more slowly than metal;
- Slag is lighter than metal.
What does this all mean? In fact, everything is very simple, and during the welding process it is not difficult to distinguish slag from metal.
Just take a closer look at the process itself, and it will become clear that the slag often tends to escape into the weld pool; it is dark in color and is easily blown away if the angle of movement of the electrode is changed. You should always remember that slag should not get into the weld pool, and this is the most important rule when welding metals.
How to remove slag during welding
To completely expel slag during welding, it is enough to maintain a large inclination of the electrode. By the way, this will make it easier to recognize this very slag from the metal. It is important to note that a large inclination of the electrode can blow off not only slag, but also hot metal, so it is important to find a “golden mean” here.
For novice welders who do not yet know how to distinguish slag from metal, the electrode must be held vertically while welding. By making small oscillatory movements with the electrode across the seam, it is necessary to achieve, thereby, overlapping the edge of the chamfers. You can also try welding at an angle backwards, this way it will be much easier to distinguish slag from metal.
And lastly, when welding with electrodes with a basic coating, it is much easier to recognize slag. It can be driven out either by the electrode itself (by changing its tilt), or by increasing the current strength or by using a short arc. Don't be afraid to experiment, and you'll probably succeed!
Source: https://mmasvarka.ru/kak-otlichit-shlak-ot-metalla-pri-svarke.html
Welding slag: how to distinguish it from metal when welding | mk-soyuz.rf
When welding, a black loose crust is formed over the place where the metal is joined (seam) - welding slag. It consists of molten flux or electrode coating, oxidized metal. In the process of melting and joining the materials to be welded and the electrode, it closes the melting bath. Thanks to the slag, the seam cools slowly, without oxygen, and does not oxidize.
Once molten waste gets directly into the seam, it causes rapid destruction of the metal. You can prevent oxides from getting into the seam by changing the welding modes and position of the part.
To prevent leakage and clogging of the seam, you need to know how slag differs from metal when welding. As a rule, these are materials of different density and viscosity that heat up at different rates. At the beginning of welding, the metal begins to melt and turns red. After welding is completed, it cools down faster and darkens. The slag is blown out of the bath onto the surface. At the beginning of work it is dark and poorly heated. It cools down more slowly and becomes lighter than the metal.
By cleaning the seam and tapping it with a hammer, it is easy to recognize the metallic sheen of clean steel and black matte inclusions.
There is no pure iron in slag; it consists of oxides that are formed during the melting of metal and flux. The composition varies slightly depending on the coating of the rod, but basically consists of the same substances. The table shows data on the 3 types of electrodes used most frequently:
| Substance, oxide | , % SSSI | , % OMM-5 | , % C-3 |
| gland | 7,9 | 13,2 | 18,5 |
| titanium | 2,2 | 15,2 | 12,2 |
| manganese | 4,6 | 28,9 | 13,7 |
| calcium | 42 | 3,6 | 8,1 |
| silica | 43,3 | 39,1 | 47,5 |
The composition depends on the material of the rod itself and the coating. Iron oxide is partially obtained as a result of contact of the material with air at high temperatures.
Welding slag as a prerequisite for the occurrence of welding inclusions
At the initial stage, the oxides protect the hot metal from rapid cooling and close the seam from above, blocking the access of air. Then they must be removed to check the quality of the connection, the presence of defects in the form of non-metallic inclusions, tops.
When welding parts of large thickness, several seams are applied in succession. Uncleaned slag will prevent normal contact and extinguish the arc. It will remain in the form of non-metallic inclusions - defects.
The main reasons for the appearance of slag inclusions
When the weld seam is rapidly cooled, the slag does not have time to come out and solidifies in the metal. The reasons are:
- low voltage,
- slag flowing from the bath in front of the electrode,
- incorrectly selected diameter,
- rusty and dirty metal,
- uneven movement of the tool.
The operating voltage is indicated on the package with electrodes. It can be exceeded a little, then the boiling metal in the bathroom will throw out slag. It is necessary to select the correct position of the metal to be welded, lift it from the end of the seam. Do not hold the electrode strictly perpendicular, but tilt it 10–15⁰ towards you.
The metal at the welding site should be shiny. It must be cleaned of household and industrial waste, scale, and oil. They interfere with the contact of the electric arc with the steel, impairing the heating and melting of the material in the bath.
To slow down the cooling of the weld and allow the slag to escape freely, large parts and alloyed metals are preheated to 200–400⁰. This relieves stress and allows steel with high carbon and alloy content to be welded.
Why does a lot of slag form when you cook with electrodes?
During the welding process, the electrode coating or flux dissolves. Some of the iron, sulfur and phosphorus from the material being welded burns out. The electrical conductivity of slag is less than that of metal, so it heats up worse and extinguishes the arc. Increased voltage and incorrectly selected operating parameters, electrodes contribute to metal burnout, oxidation of iron and other steel elements.
The reason why there is a lot of slag lies in its structure. All substances formed as a result of metal melting from an electric arc are lighter than steel, do not have strong bonds with each other and float to the surface.
Welding with an inverter for beginners, how to weld without slag inclusions
The inverter turns alternating current into direct current and allows you to change the polarity. Reverse current welding usually produces less slag.
The advantages of an inverter over a conventional welding machine are its small size and operation on current with a household voltage of 220 W and a frequency of 50 Hz. For beginners, it is important to be able to smoothly change the current strength.
Inverter devices have additional arc functions:
- easier ignition,
- emergency shutdown when stuck,
- afterburner ignition.
All of them simplify the work of an inexperienced welder and prevent sticking of the electrode.
How to get rid
You can get rid of slag when welding with an inverter by changing the polarity of the current and moving the electrode from minus to plus. When working with thin metal, it is necessary to take into account its rapid cooling and do not apply high voltage; the sheet may burn out.
You can’t stay in one place for a long time; you need to move the arc evenly and quickly. A terminal from “–” is attached to a thin sheet, and “+” is applied to the electrode.
The thick sheet heats up and cools down longer so that the slag has time to come out, minus is applied to the metal being welded, and plus is applied to the electrode.
Source: https://xn----ntbhhmr6g.xn--p1ai/metallyi/prichinyi-poyavleniya-bolshogo-kolichestva-svarochnogo-shlaka
